1. Field of the Invention
This invention relates to a method and apparatus for locating defective fuel elements that are used in the core of a water moderated nuclear reactor.
2. Background of the Invention
There are several methods employed in the art for detecting defective fuel elements in water moderated nuclear reactors. The two most common methods are known as wet and dry sipping. Wet sipping is based upon the leaching of fission products (primarily iodine and cesium) from defective elements into an isolated volume of coolant. In a boiling water reactor (BWR) the primary advantage of the wet sipping method is that it can be used without removing fuel from the core. This is possible since in a BWR fuel elements are arranged in bundles that are surrounded by a channel that is open at the bottom and the top. Partial isolation of the fuel is obtained by placing a loose fitting cap over the fuel channel and forcing air into the cap until the coolant water is displaced slightly below the top of the channel. After a sufficient time water samples are drawn from the capped channel and measured for the concentration of fission products. This method suffers from the disadvantage that the bottom of the fuel channel is still open and the concentration of fission products leached from the fuel is reduced by both convection and a decrease in the density of the isolated water due to reduced cooling and an increase in temperature.
Pressurized water reactors (PWR) are usually operated without fuel channels thus requiring the removal of fuel elements from the core for testing. This is accomplished by lifting the fuel elements out of the core and placing them in a sealed container filled with water. After a sufficient time, the water is tested for the presence of certain leached radioactive fission products. It has been demonstrated that the sensitivity and reliability of wet sipping is increased measurably by thus removing the fuel from the core and isolating each assembly in a sealed container. Thus, with the increasing interest in locating as many fuel defects as possible the current trend in both PWR's and BWR's is towards sipping from sealed containers. However, wet sipping in general suffers from the major disadvantage that there is a rapid decrease in leachable fission products following plant shutdown. This decrease is exponential with time and since leaching is the controlling process rather than radioactive decay, wet sipping operations must be completed within a few weeks of plant shutdown for the most reliable results.
A newer method coming into use both in PWR's and BWR's is the dry sipping method. Dry sipping depends upon the expelling of fission gases through defective fuel cladding with high decay heat temperatures. The fuel is removed from the core, placed in an open bottom cylinder and the coolant is displaced to expose the fuel elements to air and obtain the required temperature increase. When the water is allowed to re-enter the container, it expels the air which is then sampled and measured for fission gas content. Signals from dry sipping have been shown to be several orders of magnitude greater than those from wet sipping and have been readily observed several months after plant shutdown. However, dry sipping suffers from the disadvantage that overheating of the cladding surrounding the fuel element is possible.
The prior art has made attempts to improve upon the wet and dry sipping methods. Specifically, U.S. Pat. No. 3,419,467 Holzer et al. discloses a method of testing fuel elements from the core of a nuclear reactor that comprises: sealing the fuel elements in a test chamber filled with water, repeatedly changing the pressure and temperature of the water in the vessel, so as to first drive water into the fuel defects and then cause it to be expelled, rinsing the fuel elements, and measuring the concentration of fission products contained in the rinse water. Holzer et al discuss the possibility of rinsing with a gas, but it is apparent that the method they employ would be subject to the same safety consideration presented with the dry sipping technique, namely being that of overheating of the fuel cladding.
The present invention has as its main object to provide a method and apparatus for detecting failed fuel elements from either a BWR, PWR or any water moderated reactor that provides the very high, and therefore very sensitive, signal found in dry sipping with the safety of wet sipping.